Method and apparatus for recording with a magneto-optical recording medium applying one period of an alternating magnetic field to a unit domain length

ABSTRACT

A magneto-optical recording medium includes a recording layer and a reproducing layer respectively formed by magnetic layers on a substrate. A record magnetic domain is formed within the recording layer by using a magnetic head, which is transferred into a reproducing layer by irradiating a laser beam upon reproduction. The physical length in recording a unit bit is taken as a unit domain length. Where the unit domain length is 1T and “1” is recorded in 1T, “1” is recorded in a former half 1T/2 and “0” is in a latter half 1T/2 by applying one period of an alternating magnetic field to the unit domain length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for recording with amagneto-optical recording medium. More particularly, the inventionrelates to a method and apparatus for recording with a magneto-opticalrecording medium, wherein the recording medium includes a recordinglayer and a reproducing layer for recording magnetic domains within therecording layer upon recordation and transfer of the record domains tothe reproducing layer upon reproduction.

2. Description of the Prior Arts

The magneto-optical recording mediums and recording/reproducingapparatuses of this kind are disclosed, for example, in Japanese PatentLaying-open No. H6-295479 (Oct. 21, 1994) G11B 11/10, Japanese PatentLaying-open No. H8-7350 (Jan. 12, 1996) G11B 11/10, and so on.

The magneto-optical recording medium 10 includes, as shown in FIG. 1, arecording layer 14 and a reproducing layer 16 that are formed by amagnetic layer on a substrate 12. An intermediate layer 18 is formedbetween the recording layer 14 and the reproducing layer 16, while aprotection layer 20 is provided on the recording layer 14. Theintermediate layer 18, although formed herein by a non-magnetic layer,may be formed by a magnetic layer. Meanwhile, the recording layer 14 andthe reproducing layer 16 may be formed by an arbitrary known magneticmaterial. Referring to FIG. 2, microscopic domains 22 can be recordedwithin the recording layer 14 of the magneto-optical recording medium 10by using a magnetic head (not shown). During reproduction, the recorddomain 22 is transferred from the recording layer 14 to the reproducinglayer 16 by irradiation of a laser beam 24 as shown in FIG. 3.

More specifically, a temperature profile is given in the magneto-opticalrecording medium 10 by irradiating the laser beam 24 as shown in FIG. 3.The temperature is highest at around a spot center and graduallydecreases as an outer side is approached. Note that, in the case wherethe magneto-optical recording medium is for example a disc, thetemperature profile is different in slant at between the front side andthe rear side with respect to a moving direction of the magneto-opticalrecording medium. The temperature gradient is more abrupt at a region ofthe disc coming into a laser spot than that of a region going out of thelaser spot. The magneto-optical recording medium 10 is increased intemperature at a desired point by utilizing such a temperature profile.

Returning to FIG. 2(A), if a laser beam 24 is irradiated to themagneto-optical recording medium 10, the magneto-optical recordingmedium 10 is increased in temperature to provide such a temperatureprofile as shown in FIG. 3. Here, the reproducing layer 16 is formed bya magnetic layer which is rich in transition metal and assumes a form ofa magnetic thin film with perpendicular magnetization within a rangefrom the room temperature to the Curie temperature Tc. As a consequence,the reproducing layer 16, if irradiated by a laser beam 24, is raised intemperature and decreased in coercive force. Due to this, theirradiation of the laser beam 24 causes the reproducing layer 16 to risein temperature and hence decrease in coercive force, so that the recordmagnetic domain 22 of the recording layer 14 is transferred through theintermediate layer 18 to the reproducing layer 16 by the action ofstatic magnetic coupling, thus forming a transferred magnetic domain 26within the reproducing layer 16. The transferred magnetic domain 26 isformed at a position corresponding to the record magnetic domain 22.

After forming the transferred magnetic domain 26 within the reproducinglayer 16, an external magnetic field Hep is applied by a not-shownmagnetic head as shown in FIG. 2(B). This external magnetic field Hep isan alternating magnetic field. At least one period, preferably 2 to 4periods, of an alternating magnetic field is applied during each timeperiod that one magnetic domain passes through a hot spot 24 a (see FIG.3) caused by the laser beam 24. If an alternating magnetic field orexternal magnetic field Hep is applied in the same direction (samepolarity) as that of the transferred magnetic domain 26, then thetransferred magnetic domain 26 is increased in diameter to have enlargedmagnetic domains 26 a and 26 b. As a result, transfer of the recordmagnetic domain 22 is effected with magnification. If a laser beam forreproduction is irradiated to the transferred magnetic domain 26 as wellas to the enlarged magnetic domains 26 a, 26 b by using the optical head(not shown), reproduction is made of a magnetization state or recordsignals from the reproducing layer 16.

There is known one approach to realize high density recording, in themagneto-optical recording medium and recording/reproducing apparatus ofthis kind, wherein record magnetic domains are provided different indomain length 1T, 2T, 3T, . . . , as shown in FIG. 4.

In this conventional recording method, however, there encountersvariation in a state of a leakage magnetic field passing through thereproducing layer of the magneto-optical recording medium due todifference in domain length. Thus there has been a problem that theoptimal reproducing condition is different for each domain length thusresulting in worsened reproducibility.

More specifically, if considering a long domain as shown in FIG. 5(B),the reproducing layer has a leakage magnetic field that is strong at adomain end P1 but weak at a domain central region P2. Meanwhile, throughan outside point P3 of the domain is caused a leakage magnetic field ina direction opposite to that of the domain end P1. In such a state, ifan external alternating magnetic field be applied, the leakage magneticfield at the domain outer point P3 acts to prevent the magnetic domainfrom being transferred into and enlarged within the reproducing layer toa satisfactory extent.

On the other hand, where the domain is excessively short as shown inFIG. 5(A), the leakage magnetic field is less distributed throughout thedomain. There is also reduction in the opposite directional leakagemagnetic field at the domain outer side area. Accordingly, theapplication of an external alternating magnetic field causes themagnetic domain to be transferred to and enlarged in the reproducinglayer with sufficiency.

It is therefore difficult, for the conventional high-density recordingmethod to record by varying the domain length, to obtain a reproductioncharacteristic with uniformity, because of uneven transfer andenlargement of the magnetic domains into and within the reproducinglayer due to the difference in domain length.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide amethod and apparatus for recording with the magneto-optical recordingmedium.

It is another object of the present invention to provide a method andapparatus for recording with a magneto-optical recording medium whichcan stably reproduce under a same condition signals having been recordedby changing the domain length.

The present invention is a method for recording with a magneto-opticalrecording medium having a recording layer and a reproducing layer formedas a layer on a substrate, comprising the step of: recording a signalonto one part of a unit domain length (1T).

An apparatus for recording a signal on a magneto-optical recordingmedium according to the present invention, comprising: a modulatingmeans for modulating a record signal; a timing signal creating means forcreating a first timing signal based on the record signal modulated bythe modulating means; and a magnetic field applying means for applyingone period of an alternating magnetic field to a unit domain length inresponse to the first timing signal.

The physical length for recording a unit bit is taken as a unit domainlength. In the case that the unit domain length is 1T, a signal “1” isrecorded, for example, in 1T/2. More specifically, one period of analternating magnetic field is applied to the magneto-optical recordingmedium during a time period of the unit domain length 1T. Accordingly,recording a signal “1” of 1T requires to record “1” in the former 1T/2and “0” in the latter 1T/2. Recording a signal “0” of 1T requires torecord “0” in both the former 1T/2 and the latter 1T/2. To record asignal “1” of 2T requires recording twice “1” and “0” alternately at a1T/2 interval.

Because the substantial domain length is limited to 1T/2, thereproducing condition may be optimized only for the domain length of 1T/2. Also, there is less distribution of a leakage magnetic field throughthe domain, and there is reduction in an opposite directional leakagemagnetic field that is formed at the outer side of the domain.Accordingly, it is possible to transfer and enlarge the magnetic domainto and within the reproducing layer in a sufficient extent.

According to the present invention, because the substantial domainlength is taken short, even if the domain length is varied, the transferand enlargement of the domain to and within the reproducing layer ismade with sufficiency, thus realizing stable reproduction. Because thedomain length is limited to one kind, it is possible to widen a marginfor the reproducing condition.

The above described objects and other objects, features, aspects andadvantages of the present invention will become more apparent from thefollowing detailed description of the present invention when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional illustrative view showing one example of amagneto-optical recording medium used in the present invention;

FIG. 2 is an illustrative view showing a conventional method toreproduce magnetic domains recorded in a recording layer of themagneto-optical recording medium of FIG. 1, wherein

FIG. 2(A) is before enlargement while

FIG. 2(B) is after the enlargement;

FIG. 3 is an illustrative view showing a light spot with temperaturedistribution given by a laser beam irradiated upon reproducing with amagneto-optical recording medium;

FIG. 4 is an illustrative view showing a conventional high densityrecording method having domains varied in length;

FIG. 5 is an illustrative view showing typically a leakage magneticfield occurring in a reproducing layer in the conventional part of FIG.4, wherein

FIG. 5(A) illustrates a case of a short domain length while

FIG. 5(B) a case of a long domain length;

FIG. 6 is an illustrative view showing a domain to record “1” in a unitdomain length according to the present invention;

FIG. 7 is an illustrative view showing a domain to record “0” in theunit domain length according to the present invention;

FIG. 8 is an illustrative view showing domains to record “1” in a domainlength of 3T according to the present invention;

FIG. 9 is a block diagram demonstrating one embodiment of the presentinvention;

FIG. 10 is a timing chart showing the operation of an externalsynchronous signal creating circuit in the FIG. 9 embodiment;

FIG. 11 is a graph showing a bit error rate against an intensity of anexternal magnetic field when recording according to the conventionalpart;

FIG. 12 is a graph showing a bit error rate against an intensity of anexternal magnetic field when recording according to the embodiment ofthe present invention; and

FIG. 13(A) is an illustrative view showing one example of domainscreated according to the conventional art while

FIG. 13(B) is an illustrative view showing one example of domainscreated according to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 illustrates a method to record a signal “1” onto a unit domainlength 1T . In this invention, the unit domain length is divided torecord “1” in the former half and “0” in the latter half of the divideddomain. In this case, a “+” magnetic field is applied to the former 1T/2while a “−” magnetic field is to the latter 1T/2 by applying one periodof an alternating magnetic field onto the unit domain length 1T throughusing a magnetic head 36 (FIG. 9).

FIG. 7 similarly illustrates a method to record a signal “0” onto theunit domain length 1T , wherein “0” is recorded in a former half and “0”is in the latter half of the divided domain.

FIG. 8 depicts a method to record a signal “1” within a domain length3T. In this invention, “1” and “0” are alternately recorded at aninterval of 1T/2. That is, the domain length 3T is divided into sixportions each having a domain length 1T/2, thus making recording as “1”,“0”, “1”, “0”, “1” and “0”. In also this case, one period of analternating magnetic field is applied to each unit domain length 1T .

Note that the length 1T/2 corresponds to a physical length of 0.08 μm onthe magneto-optical recording medium 10. The magneto-optical recordingmedium, if having a diameter of 12 cm, can afford to record signals asmuch as 10 to 12 giga-bytes.

Referring to FIG. 9, a record reproducing apparatus 30 applicable forthe magneto-optical recording medium of this embodiment includes aspindle motor 32 to rotate a magneto-optical recording medium or disc10. The spindle motor 32 is controlled by a servo circuit 34. A magnetichead 36 is provided above the magneto-optical recording medium or disc10, in a manner out of contact with the disc 10. An optical head 38 isarranged below the disc 10 in a manner out of contact with the disc 10.The magnetic head 36 is utilized not only to create record magneticdomain 22 (FIG. 2) within a recording layer 14 (FIG. 1) of the disc 10as referred to later, but also to apply an alternating magnetic field inorder to extend a magnetic domain 26 that has been transferred to thereproducing layer 16. The optical head 38 includes, as well known, alaser element, a light detecting element and a polarized light beamsplitter. The laser element (not shown) is to irradiate a laser beam tothe magneto-optical recording medium or disc 10 during reproduction, asstated before. Two light detecting elements, e.g., photodiodes, serve todetect respective ones of reflection light different in polarizationaxis in accordance with a magnetization polarity of the record magneticdomain or transferred magnetic domain (enlarged magnetic domain),thereby outputting a reproduction signal (RF signal).

The reproduction signal by the optical head 38 is supplied to areproduction signal amplifying circuit 40. The reproduction signalamplifying circuit 40 supplies tracking error and focus error signalscontained in the reproduction signal to the servo circuit 34. The servocircuit 34 in turn controls the spindle motor 32 to rotate at apredetermined rotational speed in response to the tracking and focussignals as well as a clock signal (to be referred to later). The servocircuit 34 also controls an objective lens (not shown) included in theoptical head 38. That is, the servo circuit 34 performs tracking servoand focus servo controls.

The reproduction signal amplified by the reproduction signal amplifyingcircuit 40 is removed of noise components by a low-pass filter 42, andthen supplied to a PLL (Phase-Locked Loop) 44 serving as a clockgenerating circuit as well as to a decoder 46. The PLL 44 makesadjustments on phase and frequency of an oscillation clock based on aphase comparison between a reproduction clock contained in thereproduction signal and an oscillation clock given from VCO(Voltage-Controlled Oscillator; not shown), thereby outputting anoscillation clock as a system clock. This system clock is supplied tothe servo circuit 34 as stated before, and also to a control circuit 48and to the decoder 46.

The decoder 46 decodes an output signal (reproduction signal) suppliedfrom the low-pass filter 42, thus outputting reproduction data.

The control circuit 48 controls a magnetic head drive circuit 52 and alaser drive circuit 54. The laser drive circuit 54 effects driving thelaser element (not shown) included in the optical head 38, dependingupon a signal given from the control circuit 48.

The magnetic head drive circuit 52 includes a record pulse signal source(not shown) for generating a pulse signal to write record magneticdomain into the recording layer 14 (FIG. 1), and a reproduction pulsesignal source (not shown) for causing the magnetic head 36 to generatean alternating magnetic field. That is, the control circuit 48 issupplied with modulated record data from a modulator 56 so that thecontrol circuit 48 applies a signal to the magnetic head drive circuit52 in accordance with the modulated record data. In response, themagnetic head drive circuit 52 controls the pulse signal sources tosupply a drive signal to the magnetic head 36 so that magnetic domainscan be recorded within the recording layer of the magneto-opticalrecording medium or disc 10 in accordance with the record data.

Explanations will be made hereunder on the drive timing to the opticalhead 38 and the magnetic head 36 in the case where a signal or signalsare recorded in each part of a unit domain length 1T , e.g., 1T/2. Asshown in FIG. 10(A), the magneto-optical recording medium or disc 10 isformed with a land/groove schemed track. The track includesdiscontinuous sections 60 formed at a predetermined interval withoutgrooves. At the discontinuous section 60, the optical head 38 orreproduction signal amplifying circuit 40 a outputs a signal as shown inFIG. 10(B). This signal is supplied to an external synchronous signalcreating circuit 58. The external synchronous signal creating circuit 58causes a comparator (not shown) to compare the reproduction signal witha difference voltage, and outputs a pulse signal as shown in FIG. 10(C).This pulse signal is supplied to the control circuit 48. The controlcircuit 48 supplies a pulse signal, as shown in FIG. 10(E), to the laserdrive circuit 54 and the magnetic head drive circuit 52 in accordancewith the record data given from the modulating circuit 46, insynchronism with the system clock from the PLL 44 and the pulse signal.In response to this pulse signal, the laser drive circuit 54 makesdriving to the laser element (not shown) of the optical head 36 whilethe magnetic head drive circuit 52 drives the magnetic head 36.

In this manner, the present embodiment effects driving to the opticalhead 38 and the magnetic head 36 at an interval of 1T/2 based on theexternal synchronous signal obtained from the external synchronoussignal creating circuit 58.

Now the results of the experiments conducted by the present inventorswill be explained with reference to FIG. 11 to FIG. 13. FIG. 11demonstrates a reproduction characteristic (actual measurement values)of the signals recorded onto domains continuously created as shown inFIG. 13(A). In this graph, the vertical axis represents a bit error rate(BER) while the horizontal axis denotes a peak value (Oe) of analternating magnetic field applied for reproduction. For example, thevalue “150 (Oe)” on the horizontal axis represents that an alternatingmagnetic field of +−150 (Oe) is applied to reproduce a signal.Meanwhile, the white square (⋄) denotes a reproduction characteristic ofa domain in a size of 0.4 μm and the black square (▪) shows areproduction characteristic of a domain in a size of 1.6 μm. Note that,when the domain length is 1.6 μm, the interval between the domains isalso at 1.6 μm.

FIG. 12 is a graph representing a reproduction characteristic in a casewhere a signal recordation and reproduction are made according to theembodiment of the present invention as shown in FIG. 13(B).

From the comparison between FIG. 11 and FIG. 12, it can be seen that themagnetic field required for reproduction can be reduced by recording thesignal according to the embodiment of the present invention, as comparedwith the method of the conventional art. Furthermore, the shorterdomains and the longer domains can both be reproduced by a sameintensity of a reproducing magnetic field. In addition, it is alsopossible to reduce, although somewhat, the bit error rate. It wasconfirmed by these experiments by the present inventors that the presentinvention is well suited as a method to record signals.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An apparatus for recording a signal on amagneto-optical recording medium, comprising: a modulating means formodulating a record signal; a timing signal creating means for creatinga first timing signal based on the record signal modulated by saidmodulating means; and a magnetic field applying means for non-varyingapplication of one period of an alternating magnetic field to a unitdomain length of 1T in response to said first timing signal.
 2. Anapparatus for recording a signal on a magneto-optical recording medium,comprising: a modulating means for modulating a record signal; a timingsignal creating means for creating a first timing signal based on therecord signal modulated by said modulating means; a magnetic fieldapplying means for applying one period of an alternating magnetic fieldto a unit domain length of 1T in response to said first timing signal;an optical means or irradiating a laser beam onto said magneto-opticalrecording medium and detecting a reflection light beam thereof; and anexternal synchronous signal creating means for creating an externalsynchronous signal based on said reflection light beam, wherein saidfirst timing signal creating means creating said first timing signal inresponse to said external synchronous signal.
 3. An apparatus accordingto claim 2, wherein said external synchronous signal creating meanscreates said external synchronous signal based on said reflection lightbeam to be varied by a physical structure of said magneto-opticalrecording medium.
 4. An apparatus according to claim 3, wherein saidmagneto-optical recording medium has a track of a land/groove schemeincluding discontinuous sections formed by grooves formed at apredetermined interval, said external synchronous signal creating meanscreating said external synchronous signal depending upon said reflectionlight beam that is responsive to said discontinuous section.
 5. Anapparatus according to any of claims 2 to 4, wherein said externalsynchronous signal creating means creates said external synchronoussignal based on an input signal to be outputted from said optical meansdepending upon said reflection light beam.
 6. An apparatus according toclaim 1, wherein said timing signal creating means creates a secondtiming signal to make said laser beam into a pulse.